Marvelous Destroyers: The Fungus-Farming Beetles

Ecological catastrophes are unfortunate. But they can be utterly fascinating, too.

Witness the spread of so-called bark and ambrosia beetles, a collection of 7000 species whose expansion beyond their native ranges threatens trees around the world.

It's not the beetles' fault, of course. They've simply happened upon a brilliant life strategy: Rather than eating bark, which tends to be full of toxins produced by trees to discourage predation, they eat fungus that eats bark. It's one of the animal kingdom's greatest and most unappreciated symbioses.

"You know how famous leafcutter ants are because they grow fungus? Those groups evolved this just once. In the bark beetles, there are at least 11 independent emergences," said biologist Jiri Hulcr of North Carolina State University. "Go into the rain forest in South America, and you see the power and diversity of these fungus farmers. You'll barely see a tree without sawdust falling off. That's the fungus farmers at work, drilling through the trees and planting their fungus gardens."

Every group of bark and ambrosia beetles has its own unique collection of fungi, carried in specialized pockets on their bodies, in their armpits and on their backs and in their mouths, always ready to be seeded. "Chop a beetle's head off, grind it up, spread it on agar and you will see the most marvelous organisms growing from it," Hulcr said. "Those are the fungal symbionts. And they are virtually unknown."

The number of people studying the beetles could be counted on two hands, said Hulcr, who reviewed what's known about them in a paper published July 13 in Proceedings of the Royal Society B. That's not particularly suprising. Little incentive has existed to study the creatures, which for all their diversity were traditionally united by a habit of feeding on dead rather than living trees. They were important, but also innocuous and relatively inconspicuous.

Leafcutter ants are famous for farming fungus. That ability arose just once in ants. In beetles, it evolved 11 separate times.

All that changed when bark and ambrosia beetles started making long-distance trips from the locales in which they evolved.

The first known incidence of a bark beetle attacking living trees involved a species called Scolytus multistriatus and its fungal symbionts Ophiostoma ulmi and Ophiostoma novo-ulmi. They're better known as the timber import-riding agents of Dutch elm disease, which in a few mid-20th century decades almost completely eradicated the majestic trees from North America and Europe.

These are likely the tip of the iceberg. Many invasions are just now being identified, and haven't yet been described in scientific literature. Nobody knows why the beetles are suddenly attacking living trees, but Hulcr suspects it's simply an unfortunate coincidence, an evolutionary mismatch in which beetles are confused by the odor of unfamiliar trees. They think living trees are actually dead. Then the trees, unaccustomed to such attack, have either no immune defense or an exaggerated one. Like humans exposed to an unfamiliar bug, they overreact and destroy themselves.

What can be done? Not much, said Hulcr. Even if, as appears to be the case, only a miniscule fraction of foreign beetles end up causing damage, that's enough. After all, as those 7,000 species of bark and ambrosia beetles are transported around the world, there are nearly unlimited opportunities for mismatch. According to pest surveys of shipping containers at U.S. ports-of-entry, bark beetles account for 58 percent of all intercepted insects. Stopping them is practically impossible, and just a single beetle can be enough to spawn an invasion.

"Another amazing feature of these beetles is their amazing reproductive strategies," Hulcr said. Females are often capable of self-fertilization, producing fresh generations of offspring in the absence of mate. They don't even need to bother with sons, but may have only daughters, which thanks to their rich fungal diets are ready to reproduce themselves in less than two weeks.

On the subject of fungus, Hulcr is eloquent. "They smell like white fruit. They look like puffy clouds. Sometimes they look like brown sludge. They often taste like mushrooms. So no wonder the beetles like them," he said. Asked whether he'd tasted the fungus himself, Hulcr said yes. "Wouldn't it be fascinating to grow beetle symbiotic fungus on a large scale, so we could turn wood into fruit? There are so many opportunities. This is one of the most amazing systems out there. This is so cool and it's so unexplored."

On the following pages are photographs of fungus-farming beetles at work.

Above: Ambrosia Beetle Diversity

A selection of tropical Ambrosia beetles. None is more than 3 millimeters long.

Redbay Ambrosia Beetle

Closeup of a redbay ambosia beetle, cause of the lethal laurel wilt disease.

Beetle Garden

A granulated ambrosia beetle inside a tree. The white surface is fungus. To the right are eggs.

Beetle Attack

Redbay and granulated ambrosia beetles, both native to southeast Asia, attacking a tree in the southeastern United States. The white extrusions are stacks of sawdust.

Beetle Attack Cross-Section

An attacked tree riddled with beetle tunnels.

Fungus Pockets

Fungus pockets in the “armpit” of an ambrosia beetle. Red matter is fungal spores.

Extreme Sexual Dimorphism

Male and female ambrosia beetles may have radically different bodies and lifestyles, with males useful for little else than mating. At left, a female Diuncus papatrae from Papua New Guinea; at right, her brother.

Extreme Sexual Dimorphism #2

At left is a female Hadrodemius globus from Papua New Guinea. At right is her wingless brother.

Extremely Extreme Sexual Dimorphism

Using males for strictly reproductive purposes reaches its logical conclusion in Ozopemon, an ambrosia beetle family in which males grow mandibles and copulatory organs but otherwise stop developing at grub stage.

The Cycle Continues

Ambrosia beetle family inside a killed tree. The fungal garden is exhausted and blackened, but the larvae have matured and will soon emerge as adults.